A permanent magnetic electric machine is classified as a permanent sine-wave electric machine and a permanent trapezoidal-wave electric machine (brushless DC electric machine). Take a motor for example, the permanent sine-wave motor has a back electromotive force of a sine wave and fits a stator current of another sine wave for producing a constant torque. Otherwise, the brushless DC electric machine has a back electromotive force of a trapezoidal wave and fits a stator current of a square wave for producing a constant torque.
The brushless DC electric machine includes a rotor and a stator winding by at least a winding. Due to the existence of a winding inductance, a winding voltage must lead a back electromotive force by an angle known as an advance angle which is an important factor affecting the efficiency and the noise of the electric machine. An optimum advance angle is related to the rotating speed of the rotor. The higher the rotating speed is, the greater the optimum advance angle will be. A typical controller of the brushless DC electric machine does not control the advance angle, so that the performance of the electric machine is worse in the whole scope of the rotating speed.
At present, common methods are as follows. The first method uses software or a digital signal processor for controlling the advance angle and is disclosed in the US Publication No. 2006/0132076 A1. Good as the effect of this control method is, the cost is higher.
The second method uses hardware for controlling the advance angle and is disclosed in the U.S. Issuance Pat. No. 4,276,504. Please refer to FIG. 1(a), which is a schematic block diagram disclosed in the U.S. Pat. No. 4,276,504 showing a conventional system for controlling an advance angle of a brushless DC electric machine. Afterward, please refer to FIG. 1(b), which is a schematic diagram showing a relation between the advance angle and a motor rotating speed of the system in FIG. 1(a). As shown in FIG. 1(b), the used advance angle is a small fixed value while there is a low rotating speed (MOTOR SPEED<S1), and the used advance angle is a large fixed value while there is a high rotating speed (MOTOR SPEED>S1).
An optimum advance angle of the motor increases with the increment of the rotating speed in the whole scope of the rotating speed. Therefore, this control method cannot guarantee the advance angle to be optimum in the whole scope of the rotating speed. Besides, a tacho-generator is used in FIG. 1(a), which makes the structure of the control circuit complicated, so that the cost increases.
The third method also uses hardware for controlling the advance angle and is disclosed in the WO Patent No. 97/33363. Please refer to FIG. 2(a), which is a schematic diagram disclosed in the WO Patent No. 97/33363 showing a conventional circuit for controlling an advance angle of a brushless DC electric machine. Afterward, please refer to FIG. 2(b), which is a waveform diagram showing signals obtained from the circuit in FIG. 2(a). As shown in FIG. 2(b), the advance angle φ increases with the increment of the rotating speed.
However, as the circuit structure in FIG. 2(a) is a high-pass filter, the robustness of this control system is worse. Besides, only when position signals 48 and 50 are sine waves, the controlled advance angle is exact, but in general, the position signals 48 and 50 include many harmonic waves, so that the controlled advance angle is not exact.
In sum, in order to overcome the aforementioned defect, it is necessary to design a control system that the hardware circuit is stable, the cost is low, and the advance angle tends to be optimum in the whole scope of the rotating speed, which becomes the primary motive of the present invention.